What is a nova?
A
nova is a nuclear-powered eruption on the surface of a white dwarf.
A
white dwarf is a burned-out cinder of what once was a star like the Sun.
The
gravity of a white dwarf is so strong the white dwarf becomes essentially
solid, or in other words, becomes degenerate.
(Painting by Pat Rawlings,
NASA/STScI)
Gas
from a close companion star spills onto the white dwarf.
Star systems
like this are called cataclysmic
variables.
(Painting
by Pat Rawlings, NASA/STScI)
The
temperature, pressure, and degeneracy build up until the gas detonates, in a
thermonuclear runaway.
(Painting
by Pat Rawlings, NASA/STScI)
The
atmosphere of the white dwarf leaps up, expanding in a fireball, which engulfs
the white dwarf’s companion star.
(Hubble Space Telescope
images by F. Paresce.)
The
fireball expands and cools. When it becomes large enough to be resolved (seen
separate from the stars), it’s called a nova shell.
In
other words, a nova shell is the resolved nebular remnant of the nova: nebula
is “cloud” in Latin, meaning a cloud of gas in space.
(Image
of GK Persei from the National Optical Astronomy Observatories)
This
nova shell is also called “the Firework Nebula.”
Often,
nova shells break up into bright sparks, separated by thin, transparent gas.
This is due to the Rayleigh-Taylor instability, which is also why fireworks
break up into sparks when expanding into air.
Spectra
we have taken at Fresno State’s
Campus Observatory have also shown this, long before there was time
for a
nova shell to develop. This is
because lines that can only form in hot, thin gas (e.g. the “forbidden”
lines, such as [O I], [O II], and [N II]) and lines that can only form in
cooler, denser gas (such as H, He I, and Fe II) were present at the same
time.
Department of Physics, California State University, Fresno